The exhaust gas from an internal combustion engine such as automobile engine contains nitrogen oxide (NOx), carbon monoxide (CO), hydrocarbon (HC) and the like. These substances can be purified by an exhaust gas purifying catalyst capable of oxidizing CO and HC and at the same time, reducing NOx. As a representative exhaust gas purifying catalyst, a catalyst obtained by supporting a noble metal such as platinum, rhodium and palladium on a porous metal oxide support such as alumina is known.
A problem arising with such an exhaust gas purifying catalyst using a noble metal is a phenomenon that a noble metal particle moves on the support during use of the catalyst and a plurality of noble metal particles combine to undergo grain growth, i.e., a phenomenon called sintering of a noble metal. In this connection, Patent Document 1 proposes a technique where a perovskite-type composite oxide layer constituting a support is epitaxially grown from a noble metal layer, thereby forming firm bonding between the support composed of the perovskite-type composite oxide and the noble metal and in turn, suppressing sintering of the noble metal. Specifically, in this document, a crystalline noble metal layer is formed on a base material, an epitaxial support layer composed of a perovskite-type composite oxide is formed on the surface of the noble metal layer, and the stack of the noble metal layer and the support layer is separated from the base material to produce an exhaust gas purifying catalyst. Furthermore, this document describes that when the thus-obtained stack of the noble metal layer and the support layer is pulverized, a powdery exhaust gas purifying catalyst consisting of a support layer with a size of 10 to 20 nm and a catalyst layer with a size of 1 to 2 nm supported on the support layer is obtained. In this method, a powdery exhaust gas purifying catalyst is obtained by pulverizing the stack of the noble metal layer and the support layer, and therefore in the obtained exhaust gas purifying catalyst, the noble metal is supported only on the outer surface of the support.
In the exhaust gas purifying catalyst using a noble metal, it is generally preferred for the purification of an exhaust gas to highly disperse and support a noble metal on a support and thereby increase the surface area of the noble metal. However, as in exhaust gas purifying catalyst 10a shown in FIG. 6(a), when individual noble metal particles 12a initially supported on support 11 are very small, for example, at the atom level, there are the problems that noble metal particles are aggregated in an uncontrollable level during use of the catalyst to undergo grain growth and that the noble metal in a relatively oxidized state is present on the support and the catalytic activity is thereby not sufficiently obtained.
In order to solve these problems due to noble metal particles that are too small, it is known to support noble metal particles having a controlled size on a support (Patent Documents 2 to 4).
With respect to the size control of the noble metal particle, it is known to form a noble metal colloid having a controlled size in a solution and thereafter, support the noble metal colloid on a support. However, as shown in FIG. 6(b), conventional exhaust gas purifying catalyst 10b obtained using a noble metal colloid has a problem that noble metal particle 12b formed from the noble metal colloid is not fixed on the support 11, and therefore during use of the exhaust gas purifying catalyst, the noble metal particle 12b moves on the surface of support 11 and undergoes grain growth.
In this connection, in Patent Document 4, as shown in FIG. 6(c), noble metal particle 12c formed from a noble metal colloid is partially buried in support 11, thereby preventing the problem that during use of exhaust gas purifying catalyst 10c, the noble metal particle 12c moves on the surface of the support 11 and undergoes grain growth. However, in this method, the portion in the noble metal buried in the support 11 cannot contact with an exhaust gas and is wasted because of no action as an active site, and therefore the noble metal must be used in a relatively large amount.
Here, in order to prevent the noble metal particle from moving on the support surface to undergo grain growth during use of the exhaust gas purifying catalyst, it is also known to utilize chemical affinity between the support and the noble metal supported on the support. In this regard, for example, Patent Document 5 proposes a technique where the noble metal is bonded with a cation in the support through oxygen on the support surface to form a surface oxide layer, thereby inhibiting grain growth of the noble metal, and describes particularly a cation having an electronegativity smaller than that of zirconium as the cation.
Incidentally, Patent Document 6 proposes that taking into consideration the fact that in an exhaust gas purifying catalyst, a three-phase interface among an exhaust gas, a noble metal fine particle and a support particle efficiently functions in terms of purification of the exhaust gas, in order to increase the three-phase interface, noble metal ions are made to collide against the support by the Lorenz force and the coulomb force using a vacuum arc deposition apparatus and are deposited and aggregated on the support, thereby producing a generally hemispherical noble metal particle on the support.
As in Patent Document 6, in the case of depositing a noble metal particle on a support using a vacuum arc deposition apparatus, it is difficult to control the particle diameter of the noble metal. Furthermore, although the noble metal particle can be deposited on the outer surface of the support, it is difficult to deposit the noble metal particle on the inside surface of the support, i.e., the surface incapable of being directly observed from outside the support, for example, the surface in a pore. Furthermore, in the production process of an exhaust gas purifying catalyst using a vacuum arc deposition apparatus, it is difficult to produce an exhaust gas purifying catalyst at a practical speed.